package examination.Grandsoft_09_15;

import java.util.Scanner;

public class Question2 {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int max = in.nextInt(), min = in.nextInt(), k = in.nextInt();
        if (max < min) {
            int tmp = max;
            max = min;
            min = tmp;
        }
        int res = 0, i = 0;
        while (i++ < k - 1){
            int cMax = max;
            int cMin = min;
            res += cMin * cMin;
            while (i < k - 1 && cMin > 1) {
                i++;
                cMin--;
                res += 1;
            }
            if (--max < min) {
                int tmp = max;
                max = min;
                min = tmp;
            }
        }
        System.out.println(res);
    }

}
